The present invention is generally directed to compositions particularly useful as marking materials in various imaging systems, and more specifically the present invention is directed to heterophase ink compositions useful in ink jet printing systems, and to processes for the preparation of these compositions. Accordingly, in one embodiment of the present invention, there are provided heterophase ink compositions comprised of polymeric substances with specific stabilizers, especially nonionic stabilizers, and oil soluble dyes. The aforementioned inks possess improved waterfastness characteristics, excellent dot and edge definition, and permit decreased ink spreading. Also, the heterophase ink compositions of the present invention enable the print quality of the images generated to be of high resolution, thus allowing these inks to be selected for plain paper making technologies.
Compositions useful in ink jet printing systems generally contain therein water soluble dyes. Thus, there is disclosed, for example, in U.S. Pat. No. 3,846,141 an ink jet composition comprised of an aqueous solution of a water-soluble dye and a humectant material formed of a mixture of a lower alkoxy triglycol, and at least one other compound selected from the group consisting of a polyethylene glycol, a lower alkyl ether of diethylene glycol, and glycerol. According to the disclosure of this patent, the printing inks viscosity is subjected to little variation with use as water is lost by evaporation during recirculation of the ink composition through the jet printer. Moreover, apparently the humectant system disclosed in this patent substantially prevents or minimizes tip drying of the printing ink in the orifice or nozzle during down time of the printer, such as when the printer is rendered inoperative. As further disclosed in this patent, the basic imaging technique in jet printing involves the use of one or more ink jet assemblies connected to a pressurized source of ink. Each individual ink jet includes a very small orifice usually of a diameter of 10 to 200 microns which is energized by magneto restrictive piezo-electric means for the purpose of emitting a continuous stream of uniform droplets of ink at a rate of 33 to 75 kilohertz. This stream of uniform droplets is desirably directed onto the surface of a moving web of, for example, paper; and is controlled to form printed characters in response to video signals derived from an electronic character generator and in response to an electrostatic deflection system; or drop on demand ink propulsion system.
Also, there is disclosed in U.S. Pat. No. 4,279,653, ink jet compositions with water soluble wetting agents, a water soluble dye and an oxygen absorber. Similarly, U.S. Pat. No. 4,196,007 describes an ink jet printing composition containing an aqueous solution of a water soluble dye and a humectant consisting of at least one water soluble unsaturated compound. Other prior art disclosing aqueous inks for ink jet printing include U.S. Pat. Nos. 4,101,329; 4,290,072 and 4,299,630.
Ink compositions for jet printing can be prepared by a number of known methods. Generally, these methods involve dissolving the various dyes, humectants, viscosity control agents, paper fixing additives, surface tension control additives, biocides and anti-oxidants in a known volume of water, followed by adjusting the pH and concentration of the solution to desirable levels. In those situations wherein the colorants selected are not water soluble, such as those containing pigments, the inks are prepared by standard known milling processes. However, these pigment dispersions are generally not sufficiently stable, accordingly when incorporated into a printing machine, the ink particles tend to agglomerate resulting in the clogging of the small nozzles present in the ink jet devices. One of the important objectives of the present invention resides in the preparation of discrete particles uniformly sized and specifically stabilized sterically, enabling the elimination of the undesirable agglomeration of these particles which are prepared by in situ polymerization techniques. Thus, for example, with the inks of the present invention the nonionic stabilizer selected is not absorbed as is the situation with known emulsion or suspension polymerization processes; accordingly, coagulation of the ink particles is substantially prevented.
Further, in U.S. Pat. No. 3,346,494 there is described a process for preparing stable microemulsions by the addition of a dispersable phase to a continuous phase to which has been added a selected combination of microemulsifiers. Examples of the continuous phase include liquid hydrocarbons such as benzene, while the dispersable phase contains a material which is insoluble in the hydrocarbon. The microemulsifier used in the process of the '494 patent is apparently comprised of three ingredients; namely, a fatty acid, an amino alcohol, and an alkyl phenol, reference the disclosure in column 1, beginning at line 64.
Additionally, polymer particles can be prepared by known suspension polymerization or emulsion polymerization processes. Both of these techniques are heterogeneous reactions in which a monomer droplet is stabilized in an aqueous dispersion medium and the particle is formed by a free radical polymerization mechanism. In suspension polymerization, there is selected a monomer soluble free radical initiator situated in the monomer droplets which are of large size, approaching 40 to 300 microns in diameter, with stabilization of the suspension being accomplished with a dispersant or suspending agent which is not considered a surfactant. With emulsion polymerization, water soluble initiators located in the dispersion medium are employed and typically ionic surfactants are used for the purpose of stabilizing the resulting dispersions. In contrast with the dispersion polymerization of the process of the present invention, the reaction mixture is initially homogeneous. Also, with the process of the present invention after initiation of the polymerization, the polymer chain grows to a particular chain length at which point it phase separates to yield nuclei which are then stabilized against flocculation by the presence of a steric stabilizer at the particle-fluid interface. One primay advantage of the aforementioned dispersion polymerization process is that it can be affected in both aqueous and nonaqueous media.
There is also disclosed in U.S. Pat. No. 4,246,154 a process for permitting the formulation of ink jet compositions from vinyl polymer lattices by a dye imbibition technique. The aforementioned anionically stabilized lattices which are obtained by emulsion polymerization are colored with a dye imbibition process. Coloring processes as illustrated in Konishiroku EP Publication No. 0.068,9003 and DE No. 3,233,555 are similar to the '154 patent ink jet procedures with the exception that there is used in the latter a preformed polyurethane latex. In contrast to these colored emulsions, which are usually stable over a narrow pH range and are extremely sensitive to the concentration of electrolyte, the process of the present invention permits colored polymer particles that, although of a similar particle size, are substantially insensitive colloidally to both pH, precipation or flocculation of the ink particles, and the concentration of electrolyte.
Further, disclosed in copending application U.S. Ser. No. 553,598, entitled Ink Jet Compositions and Processes for Preparation, the disclosure of which is totally incorporated herein by reference, is an ink composition comprised of polymers having dissolved therein oil soluble dyes; and containing surfactants, which composition is dispersed in an aqueous medium. Specifically disclosed in this copending application is an ink composition useful for jet printing processes comprised of a water insoluble polymer dispersed in an aqueous solution, the polymer containing therein an oil soluble dye in an amount of from about 5 percent by weight to about 25 percent by weight; and surfactant particles in an amount of from about 1 percent by weight to about 10 percent by weight. Additionally, disclosed in the aforementioned copending application is a process for the preparation of particles for ink jet printing which comprises (1) providing a monomer having dissolved therein oil soluble dye, (2) dissolving polymerization initiator into the monomer, (3) mixing the resulting solution with water containing a surface active agent therein, (4) subjecting the resulting mixture to ultrasound vibrations, while simultaneously or subsequently heating the mixture to affect polymerization, thereby resulting in polymer particles containing the oil soluble dye and surface active agent, which particles are essentially monodispersed and are of a diameter of from about 0.03 microns to about 2 microns.
Whereas the size of the particles produced in the process of the aforementioned copending application are determined by the mechanical homogenization process, in the present process particle sizes are dependent upon the reaction conditions selected which determines the number of particle nuclei produced from the homogeneous phase and therefore the final particle size. Stabilizers used in the two processes are also different, therefore, with the inks of the copending application, the droplets are stabilized with low molecular weight ionic or nonionic surfactants, while with the process of the present application the steric stabilizers are low, medium or high molecular weight, nonionic materials. Therefore, for example, with the inks of the present invention a larger variety of dyes can be selected because they need not withstand the oxidative environment present in the polymerizing droplets of the copending application since with the process of the present invention the dye is not introduced during the polymerization.
Although the above compositions are suitable for their intended purposes, there continues to be a need for new inks for jet printing, and processes for preparing these compositions. Additionally, there continues to be a need for ink jet compositions which when in use result in superior optical print densities, and have excellent waterfastness characteristics. Moreover, there continues to be a need for inks that contain oil soluble dyes therein, and wherein the inks are of desired particle diameter sizes. Further, there is a need for ink compositions, preferably of a diameter of 0.5 microns or less which contain an oil soluble dye therein, and are waterfast, have an excellent affinity for paper substrates and superior drying times. There is also a need for ink jet compositions with oil soluble dyes wherein the colorant can be localized therein. Furthermore, there continues to be a need for new polymerization processes, particularly processes applicable to the preparation of ink jet compositions of a suitable particle size, which processes are simple and economical in their approach. Also, in accordance with the present invention, there is provided inks desirably containing therein dyes situated, for example, in the interior of the ink particles thus enabling the dye to be chemically protected, and further preventing such materials from affecting the surface tension of the particles involved. Additionally, when these compositions impinge the paper substrate during jet printing, there is precipitated immediately on the fibers the ink particles, primarily in view of the colloidal characteristics of these particles. Accordingly, such particles separate from the colorless suspending fluid rather than undesirably penetrating into the paper. With penetration there occurs the known problem of show-through, feathering, or chromatographic separation of the dye components. In this regard, it is difficult to prepare waterfast images from water soluble dyes since the ink jet compositions are complex in that in addition to the dyes incorporated therein they contain additives to prevent oxidation thereof, additives for fixing and for controlling viscosity of the liquid ink particles, as well as additives for controlling the evaporation rate and biological activities thereof.
Also, several of the prior art ink compositions described herein are homogeneous solutions comprised of, for example, a solvent vehicle, such as water, ethylene glycol, and dimethyl sulfoxide, and a molecularly dissolved dye. The dyes selected for these inks must be of a sufficient polarity to enable their dissolution in the solvents selected. Accordingly, images obtained with these ink compositions usually have poor waterfastness characteristics. Additionally, extensive dye and solvent diffusion into the paper substrate causes undesirable ink spreading, thereby resulting in low image resolution and inferior edge acuity. The two phase ink compositions of the present invention resolve these problems.